1. Field of Invention
The present invention relates the generation of electricity through a process that converts radiative heat energy into electricity through the use of photovoltaic cells, whereby photons are translated from out-of-band spectral regions of a photovoltaic cell to in-band spectral regions of the photovoltaic cell by means of intermediary phonons located in a nano-structured solid-state heat-engine, which is based on Photonic Crystals that can separate both photons and phonons spatially and energetically and apply the optimal energy photons to a photovoltaic cell to efficiently generate electricity.
2. Prior Art
Photovoltaic cells, also called solar cells, convert light into electricity by means of an electronic band-gap, which is typically formed as a pn-junction in a properly doped semiconductor such as Gallium Arsenide (GaAs) or Silicon (Si). The pn-junction has a built-in electric potential due to the space-charge depletion zone that is found in the vicinity of the transition from p-type material to n-type material. This electric potential is the cause of a corresponding built-in electric force field that can act on charges in the space-charge depletion zone to from a current in a closed loop electrical circuit that includes the photovoltaic cell. In particular, the process is initiated by an external photon that enters the space-charge depletion zone of the pn-junction and by its proximity causes the creation of an electron-hole pair if the incoming photon at least has the energy associated with the electronic band-gap of the pn-junction. If the photon has less energy than the electronic band-gap it may pass unobstructed through the device and if it has more energy than the electronic band-gap it may give up some of the energy to quanta of lattice vibrations, called phonons, while integer multiples of the electronic band-gap energy are converted into multiple electron-hole pairs. Any energy that is converted into phonons is typically lost as thermal heating of the photovoltaic cell.
FIG. 10 shows the Power Spectral Density (PSD) 107 of the sun versus the energy of the individual photons. The shape of the distribution is a strong function of the temperature of the sun and this depends on the fuel being fused and the strength of the gravitational field of the star. Note that energies for photons are always positive but FIG. 10 also shows negative energies. These negative energies correspond to the bound states of electrons 100 in the valance band 108 of the semi-conductor of the solar photovoltaic cell. As can be seen an electron 100 that is deep in the valance band 108 can be excited from its current state to an excited state 101 in the conduction band 109. The photon that excites this transition must have energy corresponding to the energy jump 102. However, the energy 102 is greater than the band gap energy, which lies between the conduction band edge 105 and the valance band edge 106. As a result phonons may be excited from the conduction band 104 and the valance band 103 with more energy than is needed to create an electron-hole pair. This energy is lost to the heating of the material. Also photons with energies that are smaller than the electronic band gap will not interact with the device and therefore are lost as radiation that passes through the solar cell or is reflected back from the solar cell depending on the particular composition and geometry of the device. Again this is wasted energy.
In the prior art most of the incident energy is lost to lattice vibrations and therefore not converted into electricity because the electronic band-gap of the photovoltaic is relatively narrow. This is typical of the traditional single junction solar cell. To improve on the efficiency of the solar cells prior art has been developed that seeks to exploit the use of different electronic band-gaps formed by different embodiments of the pn-junction. Thus multiple electronic band-gap structures have been developed with pn-junctions with different electronic band-gaps. These are pn-junctions stacked vertically or laterally. For example it has been reported at a National Center for Photovoltaic (NCPV) conference in Denver, Colo. in Apr. 16-19, 2000 that triple vertical junction GaInP2/Ga/As/Ge concentrator cells developed by the National Renewable Energy Lab (NREL) and Spectrolab have achieved 32.2% efficiency at 47 suns and 29% efficiency at 300 suns with an Air Mass of 1.5 at 25 degrees Celsius. As the number of layers of a stacked photovoltaic device increases the mutual interference between layers, caused by lattice mismatches, increases causing increased light absorption and reduced performance of the solar cell.
In an alternative embodiment consisting of the lateral placement of different electronic band-gap photovoltaics there have been at least two approaches taken. The present author was part of the studies at NASA Jet Propulsion Laboratory (JPL) in the late 1990's that were focused on Solar Space Power (SSP). One of the efforts being developed at the time was a “Rainbow” approach that used optical components like prisms to spread out solar radiation into its component colors and then develop photovoltaics that were optimized to the specific color (energy) of the light. This proved to be difficult due to the large amount of hardware required, especially for spaceflight hardware, which is designed to be low mass.
Another prior art approach is given by U.S. Pat. No. 6,689,949, which is shown in prior art FIG. 11 and describes a light containment system that has multiple single junction photovoltaic cells that are coated with reflective type filters. Each of the photovoltaic cells shown in this prior art have different band gaps. The generation of phonons 103 and 104, as shown in FIG. 10, is avoided in FIG. 11 by using multiple reflect type filters that are on the outer surface of the photovoltaic cells in order to reject out-of-band photons. The reflected and out-of-band radiation are recycled by making multiple reflections in the containment sphere. These reflections are essentially stochastic in nature and bounce around the inside of the sphere until the photon falls on a photovoltaic cell that is matched to its energy for conversion to electricity—or is parasitically absorbed. This approach still requires that multiple designs be developed for the photovoltaic cell. Additionally, the overall efficiency will still greatly depend on the number of bounces that that input photon uses before makes. Large numbers of bounces greatly increase the probability of absorption and dissipation as heat. Finally, and most importantly this prior art is unable to exploit temperature differences to generate electricity. The prior art of FIG. 11 is restricted to light only. This precludes applications such as turning the heat energy generated by a nuclear reactor directly to electricity.
These disadvantages are overcome in the present invention.